Answer: Could you tell me what chord that F natural was part of? And the chord that followed it? Or if you have an image to attach to a reply that would help, to get the whole context. [No answer has arrived yet, so I'm going to take a stab at it in case this helps someone else].

If the F natural is part of a G chord then the next chord probably would be C, and this brief change could be analyzed as V7 of IV, IV. But it wouldn't be an actual modulation to C unless it continued for a time to cadence on C. Might just be a momentary departure...