you are not completely wrong when you say that after the pronouns me, te, se... etc you have to use de verb "être". You do use etre after those pronouns when they are part of a reflexive verb. For example:
Je me suis levé (i woke up - you do it to yourself)
Je me suis lavé (i cleaned myself)
Tu t'es demandé (you asked yourself)
As you see, with these verbs the recipient of the action is the same as the subject. Then you use the verb "être".
In your question, "je vous ai aperçue", meaning " I have seen you" - the subject "je" is different to the recipient of the action "you". Other cases where you would use "avoir" with those pronouns are:
Je t'ai demandé (i have asked you)
Je vous ai dit (i have told you)
Does it make sense? I hope it helps.