Random errors in communication via RFM12B
The preamble might require two bytes of 0xAA to be recognized and thereafter, 0x2D,0xA2 is seen and this implies that a transmission is being sent. In other words, something like 24 bits of data have to be correct. You could also argue that this might be more like 30 bits if you take into account that there might be start and stop bits surrounding each byte.

So, with random bits arriving there might be a one in \$2^{30}\$ chance that the bits "match". That is a 1 in 1 billion chance approximately. 

But you also have to consider that the receiver may only activate when there is sufficient power in a potential received transmission and this might be only 20% of the time.

This extends the probability of random noise/interference having "the right value" to 1 in 5 billion. 

If the transmission bit rate is 10,000 bits per second (approximately corresponding with a receiver bandwidth of 10 kHz) then there is about 864 million bits in one day.

So maybe every 6 days you might get noise being recognized as a potential healthy transmission.

I've just thrown numbers together knowing virtually nothing about your system. For instance the air data rate might be a lot higher than 10 kbaud or the number of consecutive preamble bytes to be recognized might only be 1. You may indeed have other problems that cause this but never rule out the random factor.