So a few weeks back I was demisting the car windows (2012 Ford Focus 1.6 TDCI) after finishing work and thought I smelt exhaust fumes in the car. Bare in mind I had the windows cracked to have a cig and the engine was on so could've easily just been residual fumes from the exhaust but nonetheless it stuck in the back of my mind and I got it checked out. Garage mechanic said he couldn't smell any fumes or find any leak to cause any smell into the cab so I thought "grand job done, nothing to worry about". But there was still something itching at the back of my mind about it so i became overly cautious when i used the AC. Then i got the smell again but it was more sweet-ish so i was looking around (as im not too mechanically minded just basic stuff) and it all pointed to a coolant leak . So, checked my levels when cold and after a drive for about 3 weeks and wasnt a change, didnt see any signs of leaking coolant or anything for that matter in the engine bay or beneath the car and now im just stumped. Now the smell is only when the engine is running, worst at idle and can barely smell if A/C button is switched off but still can smell it a small bit. Nothing when driving with A/C on and nothing whatsoever when driving or idle when A/C off. The heating in the car doesnt work unless im driving and ive seen a few post stating that that could be a cooling issue. So, could all of this be just down to a faulty A/C and just needs a service or could it be another underlying issue? Has anyone has a similar issue occur?

Ive the car booked into another mechanic in the morning to have a full going over but would love some advice on what to say to him that he should check in order to hopefully find a solution to the issue.

There is an aux heater on these.  That should run with a cold engine, cold ambient temp and the heater turned to 'hot'. 

It's essentially a toaster element so they smell like hot metal and occasionally burnt dust when running.

Odd that you say there's no heat without driving though.  That suggests the aux heater may not be working anyway.

AC gas has a distinctive chemical type smell.  If that's leaking, the AC will eventually stop chilling/drying.  No DIY test for this, you would need a garage to check gas level/pressure.

Possibly mould and bacteria on the AC evaporator.  This can be cleaned with specific foam cleaner sprayed through the vents.

Last option is heater matrix, which does smell sweet, but also makes the windows impossible to demist and coolant level should be seen to drop.

So after all that, I'm not sure what the issue is, or if there is one.