I've got a good one here!
I rebuilt the entire A/C system on my 91 Olds Toronado about 3 yrs ago. Been working fine since.
Until a few weeks ago.
It quit working and the message center said "very low on refrigerant". When it senses very low refrigerant the BCM won't let the compressor come on. So I got out my gauges and it read around 70 psi on low and hi side. So I tried putting refrigerant in and the low side quickly went up to 90 psi while the hi side stayed around 70. compressor didn't come on either.
So I thought I had a blockage between low and hi side. Figuring it was my orifice tube I had it evacuated and took the lines apart where the Orifice tube goes in, which is right before the evaporator core.
It is not there!! It'* gone!! I put my boroscope in and the damn thing is not there. And it can't get sucked into the evap core because the line makes a 90 degree bend after the orifice tube.
The boroscope won't go in any further for me to look into the evap core but I took the line off coming out of it and I'm going to put compressed air into it to see if it'* blocked up.
Another very weird problem.

Yes, that is very weird. Almost strikes me as a defective orifice tube, like slightly undersized or something.

Expansion valve?

looked again in daylight. It'* not there. I thought maybe it fell out cause when I unhooked the line there was still a bit of pressure in it and it kind of spit oil out. But I've looked around the engine and again in the lines and nothing!
I'm going to get another orifice tube and put it in. vacuum it down etc.
I'll keep you posted.