Slug in toilet cistern

There will be a gap in the wall it's coming in from I assume it's a downstairs bathroom

Hi as a quick fix , drop 2 half lemons inside the cistern......

Have the cistern inlet valve shut off and clean the cistern , also have limescale filter fitted to water incoming line( change every 2 years).

Slugs are crafty creatures they can fit through small gaps and crawl up pipework drainage maybe your cistern lid is not fitted properly small gap gaps leading through your wall or floor check carefully for any small gaps or holes cracks and seal them up this should holt the pesky so and so's but they may come under the door damp dark and cool they like it that way!??