Der Unterschied ist dir doch sicherlich klar zwischen:
- "We don't know the location of the stolen car at the present time."
- "We don't know the location of the stolen car now."
No, the difference is not clear to me. If someone intended a difference in those two sentences, it would be wise and prudent to rewrite one or both of the sentences to make the difference clear (because "at the present time" means "now.")
Same for your other set of examples.
BTW, "We don't know the location of the stolen car at the present time" looks like police-talk: police officers apparently can't use simple words because simple words would not sound "official" enough; so they add a bunch of unnecessary words.
A few months ago, in America, an inexperienced cop shot an innocent woman to death. What did the police say in their report? "We shot the woman dead"? No. They said, "The woman became deceased."