  In this work we study the information provided by a detector click on the
state of an initially excited two level system. By computing the time evolution
of the corresponding conditioned probability beyond the rotating wave
approximation, as needed for short time analysis, we show that a click in the
detector is related with the decay of the source only for long times of
interaction. For short times, non-rotating wave approximation effects, like
self-excitations of the detector, forbid a na\"{i}ve interpretation of the
detector readings. These effects might appear in circuit QED experiments.
