Agree that vs 36 is reference to the 2nd coming. The "signs" in the earlier verses are referenced to the types of things happening during the tribulation. Notice He uses the birthpangs analogy. Birth pangs for a woman produce some level of pain and discomfort, but they are only a hint of the anguish to come. Birthpangs are used by Jesus to describe all these horrible things; war, famine, earthquakes, false christs. Then comes the real bad stuff, i.e., tribulation greater than ever before.
You are still claiming that there will be definite "signs" of things leading up to Christ's return. However, Christ clearly teaches that His return will catch people off guard, as in the days of Noah.