It might have been urine.
Yup, coulda been.
Cop was gathering evidence, and what he had there was an opened bottle clearly marked “GIN” well within reach of the driver. Presumptive evidence of guilt in lieu of further investigation under chain-of-custody circumstances. Cop may or may not know what that particular brand of gin looks like. Assuming it was what the label said it was, and figuring that’s more likely than pissing in a bottle and keeping that bottle at one’s feet, safe to presume it’s an “open container” violation and time for a DUI check.
...oh, but then our possible DUI suspect tried to escape, giving the cop a half-second to decide whether & how to stop him from running people over while fleeing.