Doesn't sound like they know what they're doing. The machine will have gauges on it - I've never seen an automated machine with no indication of actual pressures even if they are meant to be idiot proof.
There won't be any power to the compressor clutch when there is no gas in the system - as the trinary switch low pressure cut off prevents the clutch engaging by breaking the circuit, to prevent the compressor engaging with no gas to move oil about the system and causing damage. It's wired in-line with the clutch, so it won't report back to the HEVAC or log any codes etc because the gas as escaped*. Chances are, its got no gas, or if it has, the trinary switch has perhaps failed.
I'd go somewhere else to start with - if they don't know why their was no power to the clutch without checking pressures, they haven't got a clue.
I can dig out the trinary switch pins to bridge if you want to check the clutch for operation etc.
*One exception - on early P38s, the HEVAC provided power to the clutch (through the trinary switch) directly, so if the system was low on gas/empty and the low pressure cut off was activated, the circuit was broken to the clutch. Because the HEVAC on these earlier models could 'see' the load/current draw of the A/C clutch, it could detect if the load was missing and throw an 'open circuit' fault on the compressor output. When this changed to drive via a relay, which draws much lower current, the error detection was programmed out. Yours will drive via a relay, and thus the error detection won't be present. Unless of course someone has swapped in an early HEVAC controller for some reason - in which case you'll find the open circuit fault for the compressor output with a nanocom. This aside, gas or no gas, correct or incorrect HEVAC, take it somewhere else.
Oh one more thing - as I'm not F Gas certified yet I had to have my RR filled up by someone else recently after the condenser leaked. I took it to my local F1 autocentre. Picked it up and was told 'oh, we've underfilled it a bit because its an older car and its a lot of strain'. For the price, I wasn't too fussed at first. But then I realised, the 1000 out of 1380 grams the Thor setup is meant to take is quite a chunk missing... and it was hissing in the evaporator really obviously, as they do when low on gas. Made them fill it properly, because frankly, the 'strain' is bullshit, and I've paid to have it filled.
The guy gave me the receipt that the machine printed out for a second recovery and refill operation... which I don't think he realised showed what it extracted - 627g. Not even close to the '1000' he supposedly put in. I know r134a is getting expensive, but I don't give a toss - I've paid to have it filled and to work properly.
The p38 takes a lot of refrigerant - if the place you go to doesn't specify that they charge by volume/weight, make sure they fill it properly and don't try to screw you over.