The hospital scene where Shane described not hearing Rick's heartbeat is the key. The producers went to a lot of trouble to calibrate the gunfire so it wasn't loud enough to cast doubt on Shane's story.
If they wanted us to believe "Shane is lying" or "Shane is deluding himself," it would have been easy just to increase the decibel level of the gunshots in the distance. Or, they could have simply not included a flashback to that scene at all, and let us wonder. Our minds would naturally wander to "Shane's got a motive to lie."
The way they chose to go with that scene tells me that they want us to infer that there really wasn't a heartbeat for Shane to hear. Whether that means Rick was dead or just in some kind of near-death zombie-coma, I dunno. But that scene will turn out to be very significant.