The Japanese language is based on relative tense. In your case, you have to choose みたいです ("seems") and みたいでした ("seemed") simply based on the time of your observation, but いない and いなかった are relative to the time of your observation.
= It looked like there was no one.
(You investigated the room a while ago and thought no one was there at that time....
Absolute times are ones that stay where they are as time moves onwards, or to think of it another way they're the ones that you can circle on a calendar or measure on a clock. So "Tuesday" or "May 25th" or "4 AM" are all absolute times, and the ever-changing now will eventually catch up to them and then pass them.
Relative times are ones measured in ...
(Disclaimer: I am a native Japanese speaker, but not an expert of language)
I feel there is a slight difference in meaning between (a) and (b).
Sentence (a) is the natural choice in most cases because of the relative-tense rule you have described. But (b) may be used to describe certain situations:
Other people ...
An absolute time or date is a time period that will refer to the same time, regardless of when it is mentioned. These are typically exact times (on the same day in the current timezone unless otherwise stated) or calendar dates. In Japanese these are usually marked with the に particle but this can be omitted in casual conversation. For example: