HideOtherBoxes is a sandbox setting in Sandboxie Ini available since v0.3 / 5.42. By default, Sandboxie enables this feature, which allows processes to be hidden from other boxes. Example of disabling this setting:
.
.
.
[DefaultBox]
HideOtherBoxes=n
Related Sandboxie Plus setting: Sandbox Options > Advanced Options > Hide Processes > Don't allow sandboxed processes to see processes running in other boxes