The most powerful radio signals leaving the Earth are the Areceibo radar transmissions, military and navigational radars, and commercial television. While none of our current SETI programs are capable of detecting these types of things from our own neighbors,that is not a reason to think that our neighbors cannot detect the signals coming from us.
All of the stars on the list below are within 20 lightyears of Earth. This means that any reasonably powerful radio,television or radar signal sent from Earth 20 years ago would be reaching the most distant star on this list right about now. This also means that any signal sent from Earth 40 years ago has had time to get to the most distant star on this list,and if anybody is home out there they have had time to send it BACK and it would be arriving HERE right about now.
In fact, we have been transmitting fairly powerful radio and television signals for a good deal longer than 40 years.
These stars are also close enough that somebody out there with a reasonably advanced technology could hit us with a fairly strong signal that might be easily detected even with simple homemade technology.
Remember too that most professional S.E.T.I. searches are done over a very narrow band of frequencies, usually somewhere in the range of the "Water Hole" frequencies of 1.4 to 1.7 Gigahertz. The Water Hole might be a reasonable place to look for deep space beacon signals, but it is not where a reply from a nearby starsystem will be heard. Our most powerful and interesting transmissions, have been on the commercial broadcast frequencies! Most notably VHF and UHF television and that in my opinion is the most likely place to look for a reply. If anyone is going to answer they would almost certainly use the same frequencies and transmission formats that they have received from us.
Look at it this way. Asking,"what might ET do to get attention?", doesn't narrow the possibilities at all. It's like not having a plan. Asking instead,"what would ET do if they picked up a TV signal from us?",leads to the obvious answer,"they would reply with the same kind of signal at a fairly strong signal level aimed at us,and if they are close enough to hear us then we can hear them". Is ET close enough to hear us? Looking only in the water hole for deep space beacons doesn't even ask the question.
To the best of my knowledge, no one has ever done a systematic, long term search of nearby stars on commercial broadcast frequencies. These signals ,if there at all, would track with the stars and would be doppler shifted. They can be seperated from similar earthly signals.How hard could it be? Would you like to organize an effort?
After all, testing a reasonable hypothesis is far better than guessing after wild speculation.Feel free to contact me.