I was once (a long time ago) a PAX on a 747 from OAK to PIT which got a fire warning light in the undercarriage area immediately on take off. Pilot didn't retract the u/c or flaps, didn't dump fuel (so far I could tell), went very slowly round the circuit (it seemed slow, and prabably was), landed very gently as soon as he got over the runway, didn't use the brakes, and let the a/c run the whole 10,000 ft to the reception committee complete in their shiny silver suits waiting at the far end of 29. Once the a/c had stopped we just sat for what seemed a long time. The PAX peered out of the windows at the suits, and the shiny silver suits peered back waiting the for the flames to show up. After 5 or 10 minutes everyine got tired of that and we taxied back to the gate. Turned out to be a false alarm, and after the FAA had signed off we were on our way after an hour or so.
My first indication that some thing was up was seeing a Flight Attendant getting a phone call and then anxiously talking to a another F/A, and both of them looking out of the window. Then I realized that we weren't climbing or retracting anything. Pilot then came on and said, "Folks, we got a red light on up here which shouldn't be on, and we are going right back to the airport. We should be back on the ground in about 10-15 minutes". He did not say it was a fire, nor that it was related to the u/c. Everything and everybody was very quiet after that.
One thing which struck me was how happily excited the flight deck crew were afterwards. It seemed to have made their day, and rightly so. They hadn't turned an incident into an accident.
