Used when someone thinks they see something unreal or non-existent, often from surprise or shock. After the magic show, I was seeing things all evening.
Pro Tip 2/3
Not Literal
The phrase isn't about actual visual abilities but perceptions influenced by mental state. I've been so sleep-deprived that I'm starting to see things.
Pro Tip 3/3
Informal Expression
Mostly used in casual conversations, rarely in formal contexts. When I saw the deal price, I thought I must be seeing things.