How about instead of describing magic items "detection" as a glowing object or whatever (unless it specifically has a property that makes it glow), the Wizard can actually feel the presence of magic. Maybe it's this pulsing in their veins or throbbing in their mind. It's connected to something, ley lines maybe, and as they get closer to the magical effect, the pulsing becomes more intense, but it's never pin-pointed with a glowing or sparkling "right here!"
Example:
There's a powerful magic ring in the pillow.
Wizard: Do I sense any magic in this room?
DM: I don't know. Give me a detect magic check.
Wizard: (player rolls and gets a high score)
DM: Absolutely. When you focus your mind, you feel this tingling in your fingertips. It starts pulsing up your arms and toward your head. You know the sensation, you can feel your magical items giving it off. But, there's another pulse not around you and your companions. It's coming from near the large canopy bed in the room.
Wizard: I move closer.
Rogue: Hold on a sec! Could it be trapped?
Wizard: Could what be trapped?
Rogue: The bed?
Wizard: I don't know. I'm just moving closer to see if the sensation grows.
DM: When you move closer to the bed, it certainly does. Now the pulse is coming in waves, like a beacon. You know it's gotta be somewhere around here.
Wizard: Hmmm. Rogue, something is here. Search this bed.
Rogue: I move up and start looking around, flipping the mattress, rooting through the bed.
DM: Sure, give me a Perception check.
Rogue: (rolls dice and scores high)
DM: Good stuff. There's nothing when you flip the mattress, just an old photograph tucked beneath it. But, when you flip the mattress, the pillow hits the ground and you hear a slight metallic sound when it makes contact with the stone floor.
Rogue: I pick up the pillow. Look inside.
DM: There's a ring in there.
Wizard: What do you see Rogue?
Rogue: I try to pocket it without the Wizard knowing. I kind of tilt the pillow so the ring gently falls into my hand. Then, I want to open up the pillow to show the wizard.
DM: Sure, give me a Thievery check. Wizard, give me a Perception check.
Rogue: (player rolls dice and scores high)
Wizard: (player rolls and scores low)
DM: Oh yeah. You got it. The ring falls in your hand. You tuck it into your belt pouch easily without the Wizard noticing.
Rogue: Ah, nothing over here. Must be hidden better than we thought or your senses are off.
DM: You guys continue off into the corridor and strangely, Wizard, you sense the Rogue pulsing slightly stronger than he usually does.
The end.