Insulin vials unexpectedly empty

Maybe somebody else has had this occur or knows what's going on. I store my insulin vials in the fridge and use them to fill pods every 3 days. Sometimes I'll put a vial with quite a bit of insulin in the fridge and then 3 days later it's empty or nearly empty. This has created some close calls and insulin is expensive so I'd rather not lose any.

Does anybody know what's going on? Evaporation somehow? Is there a way I can prevent it?