read here for chapter 20 solution

This is a very short chapter but it can be very frustrating due to
the seemingly random "You Lost Alexei" mission failed messages that
you will get. One thing is for sure, if he sees you, he will rush to
car and leave as fast as he can.

Firstly, retrieve this chapter's key from the elevator Alexei came
from. Kick off an outcrop to reach the lighting rig above and follow
it into a utility area. Have Sam cross all the gaps down to lower
section, turn left and then cross more gaps down to the power room.
There is a maintenance access tunnel entrance to the left of the
white transformer (to the right as you enter the power room). So
crouch down (X Button) and enter it (Square Button). Crawl all the
way to a small annex (Square to exit) and then enter the next short
maintenance access tunnel. It leads to the stairs that in turn lead
to the car park.

Quickly (as fast as you can while creeping) kick off the crate
pallet to reach the rig above. Follow it until you're just before
the pillar on your left and then drop down. Creep along the right
side of the Skobel Group van and then to the right, near where the
two guards (facing away from you) are. Kick off the B Lot sign to the
left of them to reach another rig. Follow this rig to another meshed
off area and drop down. Crawl under the shelf and then out through
the hole in the fence to Alexei's blue Brabus CLK.

If you are too slow doing it this way, you will lose Alexei. In that
case you're better off trying a different "riskier" method. After you
kick off the crate palette to the first rig in the car park, follow
the rig until it turns right. Up ahead is Alexei's blue Brabus CLK,
wait here until you see Alexei approach it. Then crawl back, drop
down, and carefully creep over to the right of Alexei's car.

Either way, Sam will hide in the back of Alexei's car, and presumably
do some casual hax0ring to pass the time.
::) :D ;D :o