This potion's container looks empty but feels as though it holds liquid.

Anything you wear or carry is invisible with you. The effect ends early if you attack or cast a spell.

At first, it seems quite likely that this potion vial has been emptied as you see no fluid within, but as you swish the sealed container you are convinced you feel liquid sloshing around inside.

I Drink an Potion of Invisibility
I bring the potion to my lips even though I can’t see any contents within. I take a drink that tastes like nothing but air and I wonder, for a moment, if I’ve been tricked. But I watch my hand and the bottle disappear before my eyes, and a magical veil of invisibility indeed flows over me.