Beeping sound

This morning I started my car in 27 degree weather. I let it warm up a few minutes like I always do, and within a minute a beep sound occurred. The beep happened every sixty seconds. There was no warning light on the dash except for the temperature light that goes on when the air is below 37. I turned the power off, got out of the car, walked around it, tires look OK, no fluids on the ground. Odd. After driving for about 20 minutes the beep-a-minute stopped. By that point the temperature had gone up to about 35 degrees. When I got back in the car many hours later to come home, no beeping. Any ideas? The beep is a slightly higher pitch than the seatbelt beep and happened only once every sixty seconds. Thanks,
Mary