Great question! Priests need mercy too! 

There are a few times that priests will go to Confession: 
1. Sometimes we just call to schedule an appointment with another priest. As a priest, I know A LOT of priests and have their numbers, so its easy to shoot a text and make a plan. It might even be connected with hanging out for a little while, then going to Confession at the end. 

2. Priests often help with Penance Services or retreats. Sometimes they will go to Confession before the official time begins to hear confessions of the laity. So while everyone is listening to the speaker, priests might be going to Confession backstage at the retreat or conference. 

3. The diocese schedules retreat days for priests called "Day of Recollection" and some priests are chosen to hear Confessions for any priests who want to go. 

So even if you may not see priests in line at the parish, they are going to Confession. Please pray for your priests!