E39 Coolant leak

I think I must have a coolant leak somewhere in my M52 lump in the E39.

Over the last two weekends I have had to top up my expansion tank to the Kalt/Cold level.  Not much about 50 mm level difference at the most.  So my thoughts are that there must be a leak somewhere.

While out driving on Saturday, outside temp was 2.5 deg, I had driven 6 miles, 4 miles at 70 mph.  Stopped the engine and the temp gauge was at its usual position, vertical, parked the car for 10 mins then got back in to drive another 3 miles at 70 mph.  On route I noticed the temperature gauge was now at about the 11 o'clock position i.e. the engine was colder then when I parked it.  I was surprised at this as even though the outside temp was cold, it had warmed up and normally the gauge should be at the 12 o'clock position.

Could this strange temp gauge behaviour be outlining a potential problem?

There are no leaks around any of the hose connections, nor around the stat housing nor the waterpump housing.  Heater performance is fine although I have thought it takes a while to heat up but I am putting that down to the fact that it is colder outside now.

The temp gauge always rises steadily to the vertical 12 o'clock position and stays there and has never ever gone past the vertical position.  I did once hear the thermo viscous fan kick in last month when I didn't think it should have. Is this linked to a coolant leak problem?

Could coolant be being expelled out under the expansion tank cap. Oh and before anyone asks there is no mayonnaise under the oil filler cap so I fairly sure it is not an internal leak.

I have discovered to my dismay that my car does not have a low coolant level switch in the bottom of my expansion tank. Why doesn't it have one, most cars do.  There is a space for the warning light behind the dash but there is definitely no sensor. 

The car has not had a coolant change in 7 years should it is long over due one.  Should I also take the opportunity to replace the thermostat and waterpump? 

Can anyone out there help me?

Andrew

Thanks for the tips guys. 

Update

This morning on the way to work.  Driven 12 miles on motorway at 60-70-80mph and all the time I'm thinking the car isn't that warm inside. Passenger heater at 32 and mine at 28 with fan at half speed.

Temp gauge standing to attention at the 12 o'clock position as usual.  Which indicates to me that there isn't an over cooling problem. 

Queued on slip road and temp gauge never moved.  Heater performance on above settings was poor.  It was warm but not hot.  Turned my side up to 32 and still not hot after a couple of minutes.

To me this means either over cooling but this isn't shown up on the temp gauge and or lack of coolant.  If there isn't enough coolant this to me would explain the poor heater performance.

At the weekend I'll check it over properly i.e. in daylight and with the engine running. 

Failing that I had decided to book it in for a compression test on the cooling system at the local garage as Killian has suggested.

The more I think about it I may have a pin prick hole in the rad as I wiped off what looked like dried coolant from the air filter box that may have sprayed out from a tiny orifice and then dried in a spray mark.  Hmmm.  Or was the direction of the dried spray indicating it had come from the top rad hose that is on the air box side.

Thanks once again guys

Andrew