You can be de-anonymized, even if the survey giver doesn't mean to. This happened to me once, and it was harrowing.
At the very beginning of my career, way back in 2008 and several companies ago, I worked for a big multinational company under a manager named Bob. It was my first job out of college, but my second role within the company. The job itself was a grind but I loved working for Bob. Bob was a heck of a guy and was running himself into the ground for the company. The team he managed was core to the product but, since it wasn't directly revenue producing, it was chronically underfunded and understaffed.
About six months after I started working for Bob (and, *cough*, right before the annual employee survey) the word came down that we were going through a reorg. In my exceedingly wise 22 year old opinion, Bob was getting screwed. His team was being distributed out to the various client-facing (and revenue-producing) teams, he was being left with no team or portfolio, and the work that we'd been doing on the core of the project would only be done from then on if a client directly funded it.
[N.B.: I've tried to give a flavor above of my perception of the events leading into my anonymous survey snafu. I recognize now that there could have been much more going on here. The tone of the above does not reflect how I feel about the situation now.]
The company was quite hierarchical; including Bob, the CEO, and the entire reporting chain in between, I had 17 levels of management. As a young employee, it struck me as criminally unfair that this line manager who was working his heart out lost his team and portfolio of responsibilities while the layers above him just shuffled things around.
And then, sitting among the empty cubes waiting to be reassigned, I received an email reminder that it was time to fill out the annual employee survey. The anonymous annual employee survey. Oh my.
Almost every relevant metric was a one. Job satisfaction, one star. Happiness with direction of the company, one star. Advice for management? Oh boy do I ever. Every comment box got three to ten sentences worth of rant. I complained about the 17 layers of management. I complained about everything about the company except for Bob. It was a therapeutic experience; getting that anger at the unfairness of it all out in writing let me start to adjust to the new realities of the situation. I enjoyed another week or two of slack time, was assigned to a new team, and moved on with my life.
Three Months Later...
Life on the new team was going okay. The people in the new cubes were nice enough. I was looking to leave the company, but my feelings about the company had transitioned from their nadir at a level of approximately unfathomable loathing to a more moderate, say, acute distaste. One day, our director (lets call him Randy) schedules a surprise meeting for the entire department on fifteen minutes notice, and with no title. I grab a coffee (and pay for a creamer packet because, natch, this company isn't paying for creamer for employees any more) and head in with everybody else.
The managers are there, and they are somber. It's a meeting about the annual survey results. We're going to talk through the feedback that was given on the survey. Oh yeah, I forgot about that. I wonder how this group did.
It seemed I wasn't the only person who'd given bad numerical scores. The team scores were low enough that the managers had been assigned some remediation tasks. It did seem, however, that I was just about the only person who had filled in the comment boxes. As we talked through a selection of the comments left on each item, 80% or more of them are my rant against the company from before I'd joined the team. I know from the first one that it is mine, and I am mortified.
The director read through comments aloud. There were definitely tells in mine. Some of them lined up with things I'd said after arriving on the team about screwing over really good folks. The 17 layers of management comment showed up. Lots of little hints about which clueless new grad might have authored them. About three quarters of the way through, my manager's gaze snapped to me. She'd been reading along and looking down, but some phrase had finally clicked and I knew things were going to be tough for a while.
The director wrapped up and said he was glad to speak about complaints with individuals who had them on a one-on-one basis. There were now two line managers in the department glaring at me. *Gulp*.
Things got testy after that. I assume word spread pretty quickly. After a week or two of the cold shoulder, I decided I needed to do something. It wasn't that I was being mistreated or anything; I felt terrible that in the eyes of my managers and coworkers, I'd come into their group and for no apparent reason just crapped all over everything. I also had already been accepted to a Master's program which I was intending to leave for, so I didn't feel like I had a lot to lose. So I went to the Director.
I explained that it was me who had left the feedback. I explained that it wasn't meant for him, his managers, or his organization, and that I'd left it when I was angry after the core team had been disbanded.
In a weird storybook ending, he said he agreed with a lot of it the complaints, especially (I assume) given the context. He appreciated my honesty. I got a (probably pre survey results) promotion and raise a few weeks later and left for school about a month after that.
The moral of the story is that, even aside from malicious technical measures, good old common sense, the distinct voice you write in, and the things people have heard you complain about over and over again (looking at you, fella who took away the company-provided creamer because it was too expensive) are plenty to out you in anonymous survey results. Think before you write, don't make assumptions about who or in what circumstances your comments will be read and always assume Randy will read them aloud to thirty befuddled coworkers and a couple of very unhappy managers.