I paint sump nuts/plugs with a bright luminous paint before the vehicle goes in for a full service at a main dealer. The local chap I use, I trust 100%, I know he does whatever I ask and at a very good price. However, some vehicles have to visit a franchise (main dealer).
With the plug painted it is obvious if they have removed it, marks to the paint to indicate the oil was drained, or replaced it as it won't be luminous anymore
Land Rover, Renault, Ford and Vauxhall have all been caught.
I will also fill the washer fluid to the top and tape over it with 'Do Not Fill'. Some dealers will charge up to £5 per litre of pre mixed screenwash.
The best experience was with a Land Rover dealer, not my local Oxford one I must say...I took a 2009 Range Rover in for new brake pads and discs all round. 3 weeks later it went back in for a suspension component, it had covered 288 miles in that 3 week period.
They then phoned me with the usual list of things it needed doing, and apparently the front brake pads were very low, and it would be 'dangerous' to leave it.
I duly authorised them to replace them, and went in to collect the vehicle with the previous invoice dated 3 weeks prior.
We had a nice conversation when I asked how after 288 miles they had degraded to a dangerous condition. I couldn't believe their stupidity! I refused to pay any of the invoice and left...and heard nothing from them after that.