It seems that SphereCast won't cast against triggers (even though RayCast does...)
Is there any way that I can make them work with triggers? (Maybe a layer setting or something; I am hoping here!)
A bit late maybe, but it seems that there is a workaround:
- create a new child object under your
trigger object - add a collider to the child object
that has the exact same shape as the
trigger - create a new layer mask (let’s say
“ColliderCast”) and set this layer
mask on the child collider - go to the Edit → Project Settings
→ Physics Panel - disable all collisions on the
“ColliderCast” layer mask.
Thus, nothing will collide with a “ColliderCast” object so this new collider shouldn’t change the behavior of other objects passing through.
However the sphereCast should collide with it, because it’s a collider and not a trigger (except if you filter the “ColliderCast” layer mask).
It seems to work for me.
Hope that helps. Peace 
sorry, but no the reference says you cant
however you could try a more complicated version of raycast
like having a common center and shooting a bunch of rays from it in different directions
or if you are using raycast in an update (which i dont recommend) you could lerp the direction to essentially create a sphere.
though doing it that way, i can’t foresee it being particularly useful because of how inefficeint it is
good luck