Robin2:
The OP's RX code does not use a variable called "pipe" and in the snippet you posted I think when you use the word "pipe" you are really referring to an address.
No, it's not an address but the pipe number.
bool RF24::available ( uint8_t * pipe_num )
Test whether there are bytes available to be read in the FIFO buffers.
Parameters
[out] pipe_num Which pipe has the payload available