But, external recruiters/headhunters (who may work with recruiters/HR at your current employer, or be personal friends) can see the flag. I have had an HR person at a previous employer tell me that they do know pretty well who is looking (LI is after all not the only site), but are legally forbidden to do anything about it (obviously depending on labor laws in your country).

Following the discussion on value/cost, it might be relevant to mention the freelancer's remuneration arrangements. From an employer perspective one would like to guard against an employee bolstering his/her pay at will by one-sidedly taking on more (self-created) task, then billing for those tasks. Normally some employee with (some) management responsibilities has decision making power over what this freelancer should do, for this reason. Does that exist and how does it work?

Agree with this, and feel that a more detailed paper trail capturing facts, as suggested by other answers, to back up what OP tells the boss, may be useful. Also I think it may not be a good idea to print out the Original Post and hand to the boss to read (if that was ever an idea - it may be seen as too vague and opinionated to be in tangible format that gets handed to whom knows who); if needed use it as notes but speak to the boss.

And that is what a helicopter parent gets when the helicopter child has grown up and entered the workforce...

@StephanBranczyk makes a valid point. OP should ideally at least have the counteroffer "contractually solid" before declining the original offer. Given that there usually is not much time between an offer and its deadline to accept/decline, and legal departments may not be the fastest, I wonder what would constitute legally binding (contractually solid)... But that may be another question on Law SE.

Thumbs up for (especially) code reviews (provided they are done properly and not just "rubber stamping": should be cooperative learning experience as opposed to power politics). A few years ago I joined a project on a short contract as a senior dev - new (to me) technology and concepts, made a lot of mistakes, had to learn from junior colleagues. But we had a very strict tech lead (with "difficult" personality...) that did ALL the code reviews. Got a lot of flak from him, but buckled down, did as he said, and in those few months learned SO MUCH. Will forever be grateful for that opportunity.

Just an opinion: Similarity of syntax does not equal a headstart in experience. And "environment" tools also require experience (and often differ between languages). So yeah, look at new languages (also suggested by another answer), but realize it will be a learning curve. I started in C, then did Java for many years (JRE vs binary), and now got into a full stack job where in reality 99.9% is frontend (JavaScript, Node, Angular). My experience in Clojure (which incidentally took some inspiration from JS but has a completely different syntax) helped me more than Java (or C) did.

Agreed. It might be better to be more specific about how it would impact the interview (e.g. "Sorry if I talk unclear, I had a dental procedure in hospital yesterday" or similar).

Agreed. I find it especially onerous if the assignment is machine gradable (various "code challenge" sites that also sell their services to recruiters). Candidates spend considerable time to cross the FIRST hurdle, just to be filtered out by algorithm. A company that only is interested if one can get to the right answer irrespective of the method, may not be such a good employer to work for. Also, if I were to hire juniors, I'd be more interested in their ability to learn instead of just what skills they already have - i.e. their value to me is their potential.

May differ between locales, but companies do use agents/recruiters (sometimes multiple agencies) which start their processes at different times. I've seen postings/been contacted by recruiters for jobs that had been posted month(s) before, some that I've already interviewed for, and once even for one I was already working at. So the employer itself should be the final authority (not that things are always up to date there either). Good answers below, just pointing out that things often move sloooowly between HR and the web admin, and HR and the recruiter(s).

This answer matches my experience best. On CV, I summarized the different projects (similar what was done for "regular" jobs, which could be varied esp. in contracting roles). Points that interviewers seemed to like (based on actual feedback): being able to successfully work independently, being adaptable and do work with little previous experience by independently and quickly upskilling, broad general background knowledge even if not related to role being interviewed for; moving into "how to run business", "interact with clients" territory.

@NeilMeyer I did write "not changed that much" :-) Meaning conceptually. Yeah, there's new (and useful!) additions. But analogously you also don't need to relearn English just because new words get added. Just see how it's used and start using away - or not, if it's not useful.