Michael was searched and he had no weapon on him.
The location of the meet was secret.
Nobody followed them.
The bathroom was searched by Sollozzo's man and no weapon was found.
So why would Sollozzo think that Michael came back from the bathroom with a weapon?

If anything, the fact that Sollozzo's man searched that bathroom and didn't think to look behind that tank is implausible. Either he didn't bother actually searching it, or he's the Stevie Wonder of gangsters.