Why do you bother supposing subterfuge would be involved? Someone
could just as well reinvent the patented technique independently, but
after it was patented, and then the patent holder could enforce the
patent some time later, preventing people from using the technique
without paying for it. Patented ideas should be avoided in free
software regardless of how they are introduced. Of course it is hard
to know when an idea is patented, as you say.
A patented idea can be implemented in a project haphazardly, yes, but it
would be spotty and probably easy to deal with. But a deliberate, covert
plan would provide for a systematic, mass corruption substantially more
difficult to deal with. ....