Translation:It seems as though someone is standing behind the door.
Shouldn't this translate as "It seems as though someone would stand behind the door"?
Correct. It's counterfactual: The speaker doesn't know for certain that someone is standing behind the door, so the conditional (‘Konjunktiv II’) is used. The direct way of expressing this would be ‘Es scheint so, als stünde jemand hinter der Tür.’, but in modern German the use of subjunctive in content verbs is falling out of use in favor of subjunctive auxiliary verb (‘werden’, ‘haben’, ‘sein’, and in some dialects ‘tun’) constructions.
My ear says that is not really possible in English. "It seems as though someone might be standing behind the door" might be the closest acceptable translation.
It depends on the tense. In this context, "were standing" is past subjunctive in English. By contrast, "might be standing" is present tense.