If all your code is either written in house or source you've explicitly licenced on some commercial basis then chances are there's no problem borrowing a bit from somewhere and reusing it somewhere else.
"Chances are" isn't much of a legal defense. If you're concerned about the costs of auditing and compliance for copyleft software, why not more so for software under less permissive licenses?
Firstly, if you're paying actual money for something, someone in authority has necessarily approved its use and any necessary review by lawyers should already have taken place (and this does actually happen IME).
Secondly, if you're paying actual money for the source code to a library and it wasn't very obviously provided for another reason, it is extremely likely that what you were getting for your money was the right to reuse that code as part of your own project. That's just how the industry works. It's not a 100% guarantee, of course, and standard disclaimers about consulting a lawyer apply, but even if you didn't do that the practical risk here is very low, while numerous places have got caught out when a developer "borrowed" some GPL'd code and didn't tell anyone.
"Chances are" isn't much of a legal defense. If you're concerned about the costs of auditing and compliance for copyleft software, why not more so for software under less permissive licenses?