Mairsil, the Pretender Out of Battlefield abilities
Unless otherwise specified, static abilities like the second ability of Mairsil, the Pretender function only while the card is on the battlefield.


  604.2. Static abilities create continuous effects, some of which are prevention effects or replacement
  effects. These effects are active as long as the permanent with the ability remains on the battlefield
  and has the ability, or as long as the object with the ability remains in the appropriate zone, as
  described in rule 112.6.
  
  112.6. Abilities of an instant or sorcery spell usually function only while that object is on the stack.
  Abilities of all other objects usually function only while that object is on the battlefield. The exceptions are as follows: [...]


To clear up some confusion: All abilities that aren't spell abilities, activated abilities or triggered abilities are static abilities and are subject to the rules I mentioned above. But there are some exceptions to rule 112.6 that define which abilities might be effective even outside the battlefield. Some examples are:


Characteristic-defining abilities (like "This card is all colors.")
Abilities that say that say that they function in a specific zone.
Abilities that modify the deck-building rules (like "A deck can have any number of cards named Relentless Rats.")