There are almost certainly exceptions to this, but here's what I think is a good rule-of-thumb:
If someone is on an actually physical surface/object, and is a countable noun, then you need the article. If it is a figurative use or set phrase, such as "on stage", "on ice", "on hold" etc, then there is generally no article.
So, if you go into a theatre looking for someone, and they are in the theatre setting something up on the stage, you might be told "they're on the stage". If you go in looking for someone and they're in the middle of a performance, then you might be told "they're on stage". This is complicated by the fact that theatre has lots of specialised language, and also uses "on the stage" for specialised figurative purposes as well, but we needn't get into that now.
If a project has been suspended, it might be "on ice" or "on hold", while if you're crossing a frozen pond or river, you're "on the ice". At the same time, the sport of ice dancing takes place "on ice" even though ice refers to the literal substance, because it is being used as a mass noun. A skater currently performing will be "on the ice" because it is a specific bit of ice that they are on.
I'm not aware of a figurative usage for toilet in this way, nor that toilet is ever a mass noun, so someone either using a toilet or standing on a toilet is "on the toilet".