I did had the same problem with my 2 young piraya.
After they checked their 200 l tank, with many plants and driftwood, they were never seen again...
believe me, they only ate in the nighthours, they runned for the food, picked up and,...where the hell are they?
so I decided to shoal them with 2 young RBP's.
What do you think had happened ?
The young nat's driving around the holy day, you can see them swim and eat and fight ,...
and the piraya ?
Out of sight !
I got angry after some days !
And removed the driftwood and many plants...
and now ...ah , I can see them ín action, shoaling with the nats.
But after I have lost my 2 old RBP's ( R.I.P ! thanks for 10 years !), I decided to mix the young shoal with my 2 large Cariba( in a 300 l Tank, with many plants and WOOD)...
I made the mix today , and to my horror
the 2 pirayas are gone....
The remaing P's (2 cariba, 25 cm and the young nats, 8-9 cm) shoaling togeter...
the pirayas hide behind some plants, hope to see them in the next days,( or do I have to wait the next years?)
Will look at them tomorrow, and hope to make some pics...