Could be one of the fish attacking them and killing them but then not being able to eat it.
Scarlet's are known as a micropredator. Maybe they don't like something else red in the tank. lol. Just because 2 are missing, that doesn't mean they are dead. Shrimp are GREAT hiders. I have 200+ of dark fire reds in a tank and I see 20 at a time. They are bright red and still hide. Unless you see the dead bodies first hand, don't assume they died.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps