I would recomend you to go back to the initial company policy papers (provided at the time of the initial offer) that the employee signs & 1 copy is provided to the employee & one retained with the HR. The policies should have a clause on probationary period:
There could be various scenarios to this:
1. The policies state a clear probationary period (usyally 6 months / a year). An employee ideally should be made permanent thereafter and a letter issued to the employee. In some instances, if a letter has not been issued (could be due to multiple reasons), the employee has a right to query this with the HR. Also, the employee may be deemed to be permanent if a rationale / reason has not been comunicated in writing to state otherwise.
2. There could be a specific clause on extended probation. However, a clarification can always be sought from the HR on this. I would appreciate this to be requested in writing.
3. Probation could also be delayed due to performance issues. However, this has to be clearly intimated to the employee in writing (a hard / a soft copy).
In all the above cases, an employee always has a right to demand explaination from the HR and as I previously said, I would always appreciate a written response so that I have evidence for my records.