Shower pan drains are often installed by setting the pan over a pipe stub and then inserting a seal between the pipe and the pan drain cup. This could be that. They're usually more robust, larger black rubber rings, though. See "Compression Seal" in this diagram:
Or it could be completely unrelated to the plumbing.
I would gently pop the drain grille out (removing any screws present first) and have a look for such a seal. If it's missing, seek a new replacement part before using the shower again.