Let's examine the 'some form of alien spaceship' option for a bit. This might seem fantastical, but in fact it is one of the more sensible forms of interstellar propulsion available. If we assume that alien civilisations want to travel between the stars, but they can't find any magical 'warp drives' or 'wormholes', then they would be stuck with using real-life tech. What options are there?
Rocketry would be very problematic- to get a rocket up to interstellar speeds you'd need a fuel tank about 90 times as big as the payload, even using antimatter propulsion. The problem is that you have to accelerate the fuel and propellant in a rocket as well as the payload, which makes the system very inefficient. What we need is a system where the propellant and fuel are external to the rocket, so that the ship doesn't have to carry it along. The best option is beamed propulsion.
A beam of particles carrying momentum would be aimed at the back of the spacecraft, pushing it along; this allows the spacecraft to be very lightweight as it basically carries no fuel. The spacecraft could be shaped roughly like a kite or parachute. Some speculative designs of beam-propulsion use radio-waves or microwaves to propel a very large rectenna-like grid; this may be what we are seeing.