Uplink power domain non-orthogonal multiple access (PD-NOMA) allows multiple users to simultaneously transmit their data over the same resource block by using different power levels, which are exploited by the base station (BS) for multi-user detection (MUD). Conventional PD-NOMA receivers can perform MUD when the data streams from all the multiplexed users are continuous, and the received signal is always a superposition of symbols from each user. However, the data received from each user may not always be continuous due to sporadic activity, and may contain missed packets or moments of silence. In such scenarios, the BS needs to precisely identify the users whose data packets are received at a particular time to successfully recover their messages. In this context, this letter proposes multiple detectors to efficiently detect user activity and perform MUD in such situations. It is shown through simulations that the proposed detectors significantly outperform conventional PD-NOMA receivers in terms of the bit error rate.