I'd take it to mean "not supposed to be there; from outside (the place where the reaction happened)". Webster's 1913 for "exotic" includes: extraneous. Same so

I'd take it to mean "not supposed to be there; from outside (the place where the reaction happened)". Webster's 1913 for "exotic" includes: extraneous. Same source for "extraneous" includes: without or beyond a thing; not essential or intrinsic; foreign.