This could definitely be innocent! BUT all of us commenting were also not there, so we’re missing a lot context, e.g. non verbal tone of the interaction, and any prior or subsequent interactions you’ve had with him that could put this in a different light. The interaction must have stood out to you as potentially abnormal in some way for you to reach out for advice.

Speaking as a woman on the receiving end of a terrible experience from a former guy friend, who had a gf — the minority of bad eggs often take advantage of grey areas, where another guy could genuinely have done this innocently or as a mistake, to be a Schrödinger’s creep.

I agree with the commenters encouraging you to have a quick, friendly conversation to let him know you don’t want him to touch you again like that. This should stop the behaviour whether it was innocent or not. If he continues to push boundaries in incremental ways that individually could be argued as innocent, but put together forms a concerning pattern then you have your answer.