see things
To see visual hallucinations or imagine things that are not really there. There's no dog in the back yard—you're seeing things. I could have sworn the contract said we were entitled to a lower price, but I must have been seeing things.
hear (or see) things
imagine that you can hear (or see) something that is not in fact there.ˈsee/ˈhear things
(informal) see/hear things that are not really there: So it was you that was playing the piano! I thought I was hearing things.(humorous) Tom’s washing the dishes — I must be seeing things!