Funny thing happened to my 1998 XJ last week.  I took my kids out to a place where they could ride their bikes, I stayed in the jeep keeping the engine running while watching them.

Temp slowly rose till I heard a buzzer and the 'Check Gauges' light came on.  I saw the temp (just at redline) and shut down the engine.  The vehicle was sitting in 40deg (with wind gusts) and idling for about 30+ minutes.  I've never had this problem before.

I drive the vehicle every day to work (a 15-20 minute drive mostly highway) and the temp never goes over 211 (needle fluctuates between the edges of 210).  I've recently taken several trips of over 2 hours with no problems.

Any ideas on what could be the problem?

Doh!  You're right I'll check after work and post the results...

Ok the coolant bottle is empty, and I peered into the radiator prior to starting her up and heading home, coolant looks low in the radiator.

I'm thinking I must have sprung a link.

Ok, today I had some time to check everything out.

First off I put about a gallon+ of coolant (water & antifreeze mix) into the resivoir.  That put it over the 'full' line, but I wasn't concerned.

For giggles I checked my other fluids, I was down 1/2 quart in oil (no biggie it's getting changed next week), but I was also down in Brake and Power Steering fluids (to the 'add' line).  I should check them more often than once every oil change.

I had to take my daughter to dance class - a 15 minute ride in traffic then I let the Jeep idle for a good 20-25 minutes.  Temp never went over 210, I saw no fluids leaking from under the jeep (though it does seem like everything down there looks damp from something, I'm thinking oil based?).  Both fans were on, and I squeezed the upper hose to check for pressure from the water pump and it seemed ok.

I'm still a bit concerned, the coolant went somewhere, I just haven't found where/how yet.  For now though, she's running fine.

The Jeep is almost 11 years old, and has 112K miles on it, isn't that about the time I should be thinking about replacing the water pump?  I'll see about checking the weep hole & the pully play on it this weekend.