When they use the geiger muller tube (radiation detector) they're all using it backwards, and as such, would be measuring thin air.
When Barry "Flashes" out and a moment later returns with Iris's ice cream, he must have stolen it. It would have taken longer for him to pay for it. CORRECTION: It would be easy for him to leave at least enough money behind.
When they first realize there's a problem, and Inspector Howard accuses them (incorrectly) of playing a prank, Barry and Iris have no reason to lie and go along with it like they did. This is actually counter-productive. They have no idea what happened or what's going on, there is no advantage to pretending otherwise, better for everybody to know this is a mystery to everybody. The only thing to lie about is that they're used to weird things happening, and chances are whatever happened is related to Barry being the Flash. It would be easy to admit they don't know what's happening without revealing Barry's secret or weird events aren't that surprising for them.