Usually my cover letter just has "dear hiring manager" and the job title. Someone suggested I search on sites like LinkedIn to find the name of the person who hires, and add the companies address and phone number. Is this a good idea?
No. This is bordering on cyber stalking and I wouldn’t recommend it. LinkedIn is a service meant to help individuals promote their personal brand. It's not a company directory.
Some companies have multiple offices and knowing which contact info to use would be hard.
Exactly. So why even try? Better to leave it as “Dear hiring manager” or “to whom it may concern” than to waste time trying to track down contact info for individual people within the company only to have your application delayed or ignored because you accidentally looked up the wrong person. They have people whose job it is to make sure your application gets into the right hands. It’s not something you need to be concerned about.
At one of the co-op work terms I did in school, my manager said the company proactively hides public facing information (for example they do not have a sign saying e.g. Microsoft Office here). Is this true for a lot of tech companies, that they try to limit how accessible they are to the public?
People who don’t have public-facing roles don’t have much reason to publicly list their contact info and doing so opens us up to a lot of unnecessary solicitation and other forms of harassment.
Companies tend to have systems in place to ensure that inquiries get routed to the proper individual in the proper manner. This allows people to focus on their jobs without being bombarded by emails, letters, phone calls, people showing up at their offices, etc. Could you imagine if anyone with a complaint about Excel was able to access the lead developer’s direct phone line? It would be a disaster. No one would get any work done.
I’m really glad you asked this question because there seems to be a rapidly growing idea among new job seekers that directly contacting people they think might have a say in hiring decisions over social media is a good way to help land their first gig. I find this to be very presumptuous and a bit disrespectful to those who put time and effort into developing neutral, unbiased hiring processes. Maybe this is the way things work in certain industries but I don’t think it’s a winning strategy when it comes to entry level tech jobs.