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There are situations where developers emailed/cloud uploaded/took hard drive of code they had been working on when he left the organization and were looking at prison sentences for theft...not copyright infringement. They didn't even give it to someone or reuse it somewhere. Just the fact of trying to keep it when they left as illegal.

All that to say that in some circumstances taking source code is considered theft.

Got a link?

Look up Sergey Aleynikov.

"Theft of trade secrets" is a specific legal term, not a subset of theft.

in my country, copying source code is definitely theft and is contractually enforced. but reproducing similar concepts/architectures from memory happens often

It's quite literally IP theft, not just copyright infringement.

Your jurisdiction may have a law called "IP theft" or it may not, either way the moral category of "theft" does nor apply.

(Also, if it was theft this particular example would be theft in the same way that taking a book from someone's recycling bin would be theft: no one is worse off)