the level of rx_signaldetect from GXB receiver

Hi, all, according to the handbook of stratix IV, in basic mode, rx_signaldetect is asserted when there is signal present at the receiver input buffer is above the programmed signal detection threshold, otherwise de-asserted. But in my project, I have serval receivers in basic mode, when all the inputs of receivers are floating, one rx_signaldetec is changing(sometimes high sometimes low), the others stay low. Can anybody tell me that why this happens? I think all re_signaldetect should stay low because all their inputs are floating. Any ideas? Thanks.