All the map pins disappeared (or turned gray) - did some other shopper claim them all?

Short answer: probably not - but some other shopper may have claimed a shop that put the project over the limit.

Long answer: ALL Presto Insta-Shops are controlled by limits.  When a mystery shopping company sets up a project, they can specify many different limits and quotas.  For example:

• Allow only one shop per location
• Allow each shopper to do a maximum of five shops
• Allow 100 shops total at any of 5000 locations
• Start this project on January 1st and stop it on January 31st
• Run this project until we've used up all our shopper funds

When a lot of pins suddenly disappear or turn gray, it's most likely that the mystery shopping company set up the project with more locations than shops, and the project has just met a limit (for example, the total number of shops, the number of shops at a particular chain, etc.).  For example, if the project has 5000 locations but only allows 100 shops, you'll see a lot of pins right up until the 100th shop has been claimed.  Then all the pins will turn gray - and once that 100th shop is submitted, they'll all be gone.  

It's also possible that you have hit a limit controlling your participation in the project (for example, if you've claimed the maximum number of shops allowed to any one shopper). What's not likely is that some other shopper is claiming all the shops.