So whose bug is it? The most obvious guess is the United Kingdom. As has been noted repeatedly in the aftermath of Edward Snowden's leak to Der Spiegel that the National Security Agency spies on EU facilities, this is what countries do. Secretary of State John Kerry, in response to the revelation, pointed out that such suveillance is "not unusual for lots of nations." Including the U.K. The device was found in the ambassador's office and not, say, Assange's work space, which suggests that the target may have been broader diplomatic information rather than specific details of Assange's work. And using the traditional metric of identifying suspects — motive, method, and opportunity — no one had those three things in stronger combination than the Brits.
The other obvious guess is the U.K.'s partner in international surveillance, the United States. The entire point of the Der Spiegel revelations was that the U.S. bugs foreign installations to learn state secrets. It's entirely possible that the U.S. planted the device as part of that effort. Or: Both countries worked together. Among the leaks revealed by Snowden was documentation of how the U.S. and U.K. collaborate on surveillance activity. This could be another instance in which intelligence officers from the two colluded in an effort to track the activity of a third state, one with which relations have been rather chilly. (Perhaps the most ironic outcome in this situation would be the discovery, among Snowden's leaked files, that the NSA claimed responsibility for the device.) Update, 11:00 a.m.: A spokesperson for the National Security Council had "no comment" when asked about the device.
Ecuador's political opponents extend beyond the D.C.-London nexus, of course, and it's very possible that one of its regional competitors — or any other major international power — is the culprit. While China and Russia may have less immediate interest in knowing what's going on in the embassy than does the U.S., they do have an interest. That the bug was found at all may suggest that it comes from a less sophisticated intelligence operation.
Which brings us to Ecuador. The nation has not been shy about embracing its role as burr under the West's saddle. There are few easier ways to cast aspersions on other countries than to claim the unverifiable discovery of a device that any accused country would necessarily deny. Hold up a small microphone, point at the Pentagon, and very few people would even blink an eye in assuming that the U.S. did it. As nearly everyone is, in anticipation of Patino's revelation.
That assumption may be part of the reason for the delay between the announcement of the bug and the announcement of the responsible nation. Assume it was from China or Russia or Colombia. In the current political environment, Ecuador would probably be happy to have the world assume the worst about the U.S. and Britain for a few hours. A prickly little burr, however temporary.