Tiypo
Forever Phone Posting
The smallest planet we've found in real life using this method was the size of the moon at 215 lightyears. So if you know what you're looking for, you can find pretty small objects using the method (btw Mass Relays are 15km long, the rings alone are 5km). So using much more advanced telescopes with all the data at merely 20 lightyears and knowing exactly where and when to be looking, they should be able to tell whether or not it's there.The problem is they're not dealing with a celestial body tens of thousands of Kilometers across with a mass measured in high exponents (with appropriate gravity effects). They are dealing with an object 5ish Kilometers at the longest point (and not spherical) with no mass worth mentioning. Maybe if it was active, the Eezo could have had enough exotic effects to allow for that sort of imaging.
It was itself also dark, and we have trouble spotting 5 km objects in our own system, let alone others. So I think it's a fair cop that, after about a year without notice, they lost the ability to see when it left.
They won't see details, but they'll be able to read a decrease in luminosity when it enters transit. If they get really really lucky, they'll catch when it gets removed and see a sudden spike in luminosity rather than the curve of the Relay leaving transit.