[Little Ray]

To further elaborate, unless the aliens have a magical energy source (zero point energy for example) any energy source will have IR output and, since enormous energy is required, there will be a very large IR output.

But, if the aliens have a magical energy source, then they probably have a magical FTL stardrive, too, and then, no, we won’t see them coming.

[Greysard]

But, if the aliens have a magical energy source, then they probably have a magical FTL stardrive, too, and then, no, we won’t see them coming.

Most likely the aliens would have to have that magical energy source and a matching stardrive. Otherwise travel between stars is not possible:

1. Travel at near-c speeds in normal space requires enormous energy for acceleration
2. Waste energy cannot be dumped in vacuum, other than by radiating it away - which is extremely inefficient and cannot handle large amounts of power
3. Therefore, the engine that accelerates the ship to near-c speeds must be very efficient, such as to generate very little waste energy - otherwise the engine will melt down.