The only problem with that is that in order to pull it off (and it's not really all that hard) you'd have to be really knowledgable about Astronomy, SETI, and that particular telescope. I.E., the prankster was most likely in the Ohio State program. He would have had to configure an oscillator for that freq and program it to simulate the signal characteristics the Observatory folks were expecting. Then he'd have to load it up in a car and drive it out to one of the side lobes of the big receiver (antennas are sensitive in multiple directions other than the one they're optimized for). Still, I'm enough of a romantic to doubt anyone chasing this dream would ever vandalize it that way. Astronomers don't think like hackers.

There are very good reasons for thinking the 21 cm line is where ET will be broadcasting, and expecting others to be listening. This frequency is one any spacefaring or technologically savvy civilization would already be familiar with and carefully monitoring, it is the signature of the hydrogen gas that can be used to map the galaxy. However, for that same reason, it's not a really good choice for a beacon, it forms a background hum everywhere you look, obscuring faint transmissions. I've always felt a first harmonic, say 42 cm or 10.5 cm would be less noisy. However, there may be good reasons why those wouldn't be good choices either. Perhaps the fact more people are likely to hear you there trumps the fact that band is noisy.

It's also unlikely that exact freq might also be a by-product of some ETs other activities (asteroid defense radars, normal comunications pipeline, some industrial process, or performance art). That would be stretching coincidence. A deleberate narrow band signal at that frequency would certainly be immediately interpreted by anyone who heard it as a hail, a beacon. And those originating the signal would certainly be counting on that. The early SETI pioneers got it right the first time: 1420 MHz is a very good place to transmit at, and a very good place to listen. LT Uhura knows first you use a hailing frequency, then after you make contact you switch to ship-to-ship, any Citizens Band user knows that too.

On the other hand, timing is everything. The chances anyone is listening while someone is transmitting, and that they are listening and transmitting in the right directions...

Could it be a probe, as you suggest? Yes, it certainly might. But you know, I think the most likely result of all this is that out there somewhere is a mature radio astronomer who feels very guilty now about a cruel student joke he pulled almost 40 years ago.