I have always been open to the notion of aliens having visited Earth and/or are still. However, it has conditions. For me to remain open about it it must be structured in the form of undetected. No wiggle room here. Either detected or undetected is possible, nothing in between.
Detected to me requires evidence that is convincing (to me). I do not buy into Egyptian pyramids having been, "Ancient high tech power distribution or navigation centers" (ref, Ancient Aliens episodes). I actually do not believe any evidence of detection exists. No fragments or any traces within this planet convince me. Not Nazca Lines...nor Maya/Aztec/Inca writings or relics. Nothing...nada...zip. For me anyway.
Undetected to me means possible. Who am I to say some advanced entity did not acquire the technology to remain undetected...in the past, present or future? Like a neutrino passes through our body every moment...so may they. Our ability to detect is only equal to our current, primitive in cosmos time scales, technology we have.
So. I believe we have not been visited by any alien race who has allowed us the ability to detect their visit, period.
Just my silly 2 cents!