I came across a situation where Dummy kept the card in the hand rather than placing it on the table. I must admit my concentration had wavered, so seeing a single card on the table - the suit that had been led - I took this for the lead and lost the trick as the card in the hand (but not on the table) beat mine. Is this acceptable play in Dummy and does this also apply to all other hands?
No it is not acceptable for dummy to do this. Law45B says that declarer names the card to be played and dummy picks up the card and faces it on the table.