The Continuity Alarm is a device, possibly built by Linkara, meant to help him (forcefully) stop bringing up the continuity of other comics due to the mistakes of the one he's reviewing.
Though the alarm generally goes off constantly because it seems that even comic writers can't keep their own continuity straight. Linkara does tend to have to turn it off or explain all the things wrong with the information in the comic that turned the continuity alarm on.
Dr. Linksano caused it to explode, but it was fixed since.
The alarm itself also seems somewhat self-aware and goes off when its either not supposed to or at the slightest provocation. Sometimes, however, the alarm agrees with Linkara, once delaying going off because he brought up a good point. Linkara has had issues with it leading to most of the time he turns the alarm off to try and fix it.
It was later broken during the His Blue Soul saga, though Mechakara had nothing to do with it. Initially saddened by the loss of a machine brotheren, he later saw it as a relief, as it was one less machine for Linkara to 'abuse'.
The Continuity Alarm apparently was fixed after that saga, as it later made a brief return during the One More Day review after 3 years of absence. When asked where it had been, it simply stated that it was building up the suspense for its return, though it has yet to make an appearance since the review.
Its absence is likely due to a confliction with the reviews, as it can stop the mentioning of plot points necessary to understand the context of the comics.