I have thought about the car. If you took a car out the night before or very early morning would there be any warmth left in the motor? By driving it again before the police came you would eliminate any evidence if there was more warmth in that motor than say the other car. I have no idea really how long the oil in a motor stays warm but just a thought.
Yes - though I would say after 2 hours most cars have returned to standing/non operating temperature, as well as the fluids - such as water and oils. Its variable - but it is obvious if checked.
A road driven and washed car returned and then left standing in a driveway cant un-tell that story in 30 minutes.
If the car engine was still warm and someone wanted to hide the fact that it had been used recently then the engine would have to be driven again to cover that.
Not just idling, or moved in the driveway, it would have to get to operating normal temperature and on the road.
That would cover the cleanliness (to some extent maybe) - and might cover up things like there was no dew on it (if it was a cold night), and cover why the motor was warm (if they were trying to hide that fact).
(Brain Interruption: thought... the car was taken to be cleaned elsewhere?).
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Below: Not a necessary read - skip over if you like, I wont be offended
I am skimming over this info (from my time working on cars loooong ago) - so please add as you see fit
Car engines keep their temperature for quite a while after the engine has stopped. Even if you place your hand on the bonnet or look at the temp gauge, these things wont necessarily register a heat reading, but working parts, metal casings, oils/fluids and water can be warm to the touch after an engine has been standing cold for quite a while.
Exhaust manifolds are the most exposed and hottest part of the engine when running, and heavily cast engine metals hold the most detectable heat - though where the heat is, is subjective with different engine bays, and variable because of the confined space, any inner bonnet insulation, moving air ventilation - and outside temperatures.
If a car has been run to operating temperature (normal) then stopped and let stand for 30 minutes - you could touch the bonnet and it may feel as cold as any other part of the car - but lifting the bonnet and air-feeling for any escaping heat will tell you if the car has been run. Depending on the weather temp, even after maybe 45 minutes, detectable warmth will come off the block and head part (mid to higher up) of the engine when touched - at the back of the engine closest to the fire wall (depending on the orientation of the motor) - pressure will be heightened in the radiator with hotter water - the oil on the dipstick will be warm. Gear box/transmission and fluids maybe warm ..
Basically anything that generates heat (internal metal, moving part) and disperses heat (exhaust gas outlet, fluids , oils, water - excluding fuels) will be warm at variable temperatures until the heat has dissipated completely (externally and internally) and the engine have returned to normal temperature ie: the car mechanically is cold.
So, a car can be checked to see if it has been run - and minute heat in an engine means this has happened. How long it has taken for the car to become cold completely, is the variable - but it couldn't be hidden in under an hour (IMHO) under severe scrutiny.
(I might check the times tomorrow on my car engine
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Warning: feel the air first instead of touching anything of the motor if checking for temperatures. Don't take the radiator cap off a recently run engine - water is under pressure and bl**dy hot. Use gloves or a towel if your not sure!! or ask a nice mechanic to help